Compact antenna systems are desirable for reasons such as portability, cost, and ease of manufacture, and are particularly well-suited for mobile, wireless devices. Interest in compact antenna systems has been further stimulated by the use of higher radio frequencies, for example UHF and higher, which allow for antenna lengths significantly less than 1 centimetre, and by the development of lithographic techniques which allow for antenna systems to be printed directly onto circuit boards or device housings with small form factors at low cost.
However, as the size of portable devices decreases below a quarter of a wavelength of an antenna operating frequency, it becomes challenging to provide for adequate antennas and associated ground planes embedded in the device. For example, a quarter-wavelength monopole antenna may not fit in devices below a certain size, depending on operating frequency. Adequately sized ground planes may also be too large to fit in devices below a certain size. In addition, it is difficult to provide multiple antennas with adequate isolation and/or envelope correlation coefficient in such physically small devices, for example for facilitating antenna diversity or multi-input multi-output (MIMO) communications.
Electrically small antennas (typically defined as antennas having their largest physical dimension no greater than one tenth their operating wavelength) have been proposed and used in a variety of applications, including mobile wireless devices. However, such antennas come with limitations, and it remains difficult to provide an antenna or multi-antenna system which exhibits acceptable performance for a given application, for example as measured by factors such as gain, efficiency, bandwidth, q-factor, antenna isolation, and envelope correlation coefficient.
Therefore there is a need for a compact antenna system that is not subject to one or more limitations of the prior art.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.